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Old 10-27-2010, 07:57 PM   #1
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Curious about the new v6 / exhaust diameter sizes

So basically... I've been looking at all the exhaust threads with the videos of people bolting up aftermarket stuff and it's ok - but not really that impressive.

I had a friend that went true-dual on his 3.4 v6 (OHV) and he said that in order to go true-dual and have the same flow as a 3" single, he did 2x2.25 (2-1/4) pipes and it ended up sounding buzzy and disgusting...

Perhaps true dual on the v6 just isn't all its cracked up to be? I mean..I get it for breathe-ability and all that. And there is a "cool factor" involved. But I really don't think you guys are gonna get a good sound out of the engine unless you enlarge the pipes more than 2.5". and then you will probably kill a lot of the back pressure (I could be wrong on that - depends on other variables in the engine).

Anyway - just curious as to sizes of OEM tubing vs after market stuff and how far people are going with kits (stock size vs over size).

-R

link to my exhaust in case anyone is curious:
Dyno: http://black34v6.wcfb.cc/cardyno.avi (yes its an avi..blame canon)
Normal Idle: http://black34v6.wcfb.cc/carshow.mpg

(note: the second one has a lot of the bass missing cause the vidcam wouldnt pick it up - i often get questions of "how much hp does your v8 have" and "love the sound of the SBC" after parking my car at shows)
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Old 10-27-2010, 08:03 PM   #2
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The new v6 Camaro's come with 2.25" true dual exhausts from the factory....
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Old 10-27-2010, 08:14 PM   #3
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Hrm.
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Old 10-27-2010, 08:46 PM   #4
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The exhaust from the manifolds is 2 1/2" through the h-pipe. There it necks down to 2 1/4" the rest of the way back.
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Old 10-27-2010, 09:55 PM   #5
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Quote:
Originally Posted by black34v6 View Post
Anyway - just curious as to sizes of OEM tubing vs after market stuff and how far people are going with kits (stock size vs over size).

-R
That was all you needed to say....

Anyway, I think Blade is going to a 3" setup, or maybe I have the wrong person. Somebody is/has.
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Old 10-27-2010, 10:53 PM   #6
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Nobody has 3" yet. Blade sounded like he was considering it, but prolly not. I think that it would probably be a negative idea...but thats just my hunch.
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Old 10-28-2010, 05:30 AM   #7
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Nobody has 3" yet. Blade sounded like he was considering it, but prolly not. I think that it would probably be a negative idea...but thats just my hunch.
From what I read and talking to a couple muffler shops, it would be overkill. The back-pressure would be non-existent, so the power output would be next to nothing or negative at some points on the rpm range. Also the sound would be retarded. I went with 2.5" because a .25" is usually recommended on an engine of this size. The V8 guys have encountered the same issues, they have looked at going higher than 3" but I believe some guys have scrapped the idea after trying it.

Bigger isn't always better But I think that's limited because we have NA engines, now I think if we have turbos/SC/nitrous than it could be useful, but who the hell knows, thats just a theory out of my ass
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Old 10-28-2010, 07:29 AM   #8
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I think ROD1 was the one that is actually going to do the 3" due to the N20 kit.
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Old 10-28-2010, 07:50 AM   #9
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I think ROD1 was the one that is actually going to do the 3" due to the N20 kit.
Nah........he mispoke and I got excited.......he's goin to 2.5", he picked up some stock SS pipes too
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Old 10-28-2010, 02:43 PM   #10
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Originally Posted by Blade View Post
From what I read and talking to a couple muffler shops, it would be overkill. The back-pressure would be non-existent, so the power output would be next to nothing or negative at some points on the rpm range. Also the sound would be retarded. I went with 2.5" because a .25" is usually recommended on an engine of this size. The V8 guys have encountered the same issues, they have looked at going higher than 3" but I believe some guys have scrapped the idea after trying it.

Bigger isn't always better But I think that's limited because we have NA engines, now I think if we have turbos/SC/nitrous than it could be useful, but who the hell knows, thats just a theory out of my ass
Ok, so I'm a little confused and I hope someone can explain this to me. Going with 3" pipes decreases the exhaust back pressure ("The back-pressure would be non-existent, so the power output would be next to nothing or negative at some points...")...why is this a bad thing? In my understanding of things, back pressure from the exhaust causes the engine to work harder on the exhaust stroke. We install headers specifically to reduce back pressure and to ease flow of the exhaust....correct? Headers are designed so that each cylinder has its own exhaust tube...each tube is at a specific length so that each exhaust stroke/gases hit the collector at different times to avoid back pressure...correct? All of this is to reduce back pressure. Also, if back pressure is negative, is that not better....is this not the premise of scavenging in a FI system...e.g. the turbo/FI is clearing the cylinder faster than the piston can push it out?

Does an engine use the back pressure from the exhaust for any part of the four stroke cycle....or for anything else for that matter?

If I'm being retarded....just let me know.

Last edited by PT145SS; 10-28-2010 at 03:02 PM.
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Old 10-28-2010, 03:26 PM   #11
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Destroying a myth.

Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

- Adapted from Thomas V.
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Old 10-28-2010, 03:36 PM   #12
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Ok...so maybe this answers my question:

http://www.thumpertalk.com/forum/sho...d.php?t=659727

Backpressure: The myth and why it's wrong.

I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "[COLOR=#22229c !important][COLOR=#22229c !important]Engines[/COLOR][/COLOR] need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory
Your [COLOR=#22229c !important][COLOR=#22229c !important]exhaust [COLOR=#22229c !important]system[/COLOR][/COLOR][/COLOR] is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A [COLOR=#22229c !important][COLOR=#22229c !important]4 [COLOR=#22229c !important]cylinder[/COLOR][/COLOR][/COLOR] motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust [COLOR=#22229c !important][COLOR=#22229c !important]gas[/COLOR][/COLOR] velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and [COLOR=#22229c !important][COLOR=#22229c !important]Nissan[/COLOR][/COLOR] use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?
I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the [COLOR=#22229c !important][COLOR=#22229c !important]motor[/COLOR][/COLOR] must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the [COLOR=#22229c !important][COLOR=#22229c !important]moving [COLOR=#22229c !important]vehicle[/COLOR][/COLOR][/COLOR] - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on [COLOR=#22229c !important][COLOR=#22229c !important]vans[/COLOR][/COLOR]
and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
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Old 10-28-2010, 04:20 PM   #13
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Well you can try 3" if you want, I have read those articles too, but then again you can be at the wrong end of the spectrum and over kill it. From what I read and with talks with experienced muffler shops, 2.5" seemed good for this engine. Maybe you will be the one who changes everyone's mind.
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Old 10-28-2010, 05:19 PM   #14
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Well you can try 3" if you want, I have read those articles too, but then again you can be at the wrong end of the spectrum and over kill it. From what I read and with talks with experienced muffler shops, 2.5" seemed good for this engine. Maybe you will be the one who changes everyone's mind.
I was not trying to second guess anyone, I just wanted to understand. This is a forum with all levels of knowledge and I was asking the questions so that I could learn from those that are more knowledgeable than me. At first glance, it seems as though backpressure is a bad thing and it was counter intuitive for me to think otherwise.

In fact, both articles still say that backpressure is bad to some degree. The second article articulates the fact that some resistance is a necessary evil to maintain velocity. I would not have known nor would I have thought of that unless I asked the questions. It’s also interesting to note that 3” pipes might be best if One wants the power band to be in the higher RPM range. I would not have known that either, had I not challenged you guys for a better explanation and had I not challenged myself to do more research.
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Old 10-28-2010, 06:50 PM   #15
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Originally Posted by PT145SS View Post
I was not trying to second guess anyone, I just wanted to understand. This is a forum with all levels of knowledge and I was asking the questions so that I could learn from those that are more knowledgeable than me. At first glance, it seems as though backpressure is a bad thing and it was counter intuitive for me to think otherwise.

In fact, both articles still say that backpressure is bad to some degree. The second article articulates the fact that some resistance is a necessary evil to maintain velocity. I would not have known nor would I have thought of that unless I asked the questions. It’s also interesting to note that 3” pipes might be best if One wants the power band to be in the higher RPM range. I would not have known that either, had I not challenged you guys for a better explanation and had I not challenged myself to do more research.
I don't think you were either.
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Old 10-28-2010, 06:59 PM   #16
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The way I see it Chevy had 2 1/4" pipe on it stock. I went to 2 1/2" and I'm staying there. At what point are you thinking you are going to kill the exhaust flow velocity on a non-FI small v6? I've seen big block v8's that had smaller exhaust stock. It might be how they made such a jump from stock just by opening up the exhaust and it might have been how the stock manifolds were so restrictive.
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Old 10-28-2010, 08:14 PM   #17
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Well you can try 3" if you want, I have read those articles too, but then again you can be at the wrong end of the spectrum and over kill it. From what I read and with talks with experienced muffler shops, 2.5" seemed good for this engine. Maybe you will be the one who changes everyone's mind.
My muffler guy says the same and Magnaflow backed him up when I was thinking about upping to 2-1/2". 2-1/4" maintains the torque where the 2-1/2" loses some torque but gains a little (1-3?) peak hp. And forget 3".
Want to up the torque so stayed at 2-1/4"
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